Amethyst,
Your management company is giving you the reason to reject; the credit score, while carefully not telling you to reject on that basis and instead focusing on talking about non-discrimination housing law. This makes any communication between them and you squeaky clean and proper if it were to be brought up in a court of law. They are covering their own butts as well as yours. The more I think about it the more I think they are handling it in just the right way. A proper squeaky clean response from you would be " this tenant is rejected - low credit score". The niggling bit is that your company has never sent you a credit report before in the years they've managed for you, so that would be uncomfortable if brought up in court, but it won't go that far IMO.
You are rejecting on a reasonable basis, and it costs too much to sue just on the hope of a win when the evidence is stacked against you. Don't let this sit too long.